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Controlled Auto-Ignition (CAI) combustion, also known as HCCI or PCCI, has recently emerged as a viable alternative combustion process to the conventional spark ignition (SI) or compression ignition (CI) process for internal combustion (IC) engines, owing to its potential for high efficiency and extremely low emissions. One of the most effective and practical means of achieving CAI combustion in an engine is to retain or recycle the burnt gases. In order to understand better the effects of recycled burnt gases on CAI combustion, detailed analytical and experimental studies have been carried out. The analytical studies were performed using an engine simulation model with detailed chemical kinetics. The five effects of the recycled burned gases studied include: (1.) Charge heating effect: higher intake charge temperature due to hot burned gases; (2.) Dilution effect: the reduction of oxygen due to the presence of the burned gases; (3.)

A turbocharged lean burn natural gas engine was upgraded to operate on a blend of hydrogen and natural gas (HCNG). Tests were carried out to determine the most suitable H2/NG blend for H2 fractions between 20 and 32 vol%. A 20 vol% H2 content was found to provide the desired benefits when taking into consideration the engine and vehicle performance attributes. A full engine map was developed for the chosen mixture, and was verified over the steady-state AVL8 cycle. In general, the HCNG calibration included operation at higher air-fuel ratios and retarded spark timings. The results indicated that the NOx and NMHC emissions were reduced by 50% and 58% respectively, while the CO and CH4 emissions were slightly reduced. The HCNG engine torque, power and fuel consumption were maintained the same as for the natural gas fuel. The chassis dynamometer transient testing confirmed large NOx reduction of about 56% for HCNG operation.

The purpose of this paper is to outline the effects of oxygen enrichment on the performance of a spark ignited single cylinder Ricardo research engine fueled with natural gas. Substituting gasoline with natural gas, under stoichiometric conditions, brings about a decrease in power output of a dedicated gasoline engine. This reduction in power is quantified in this paper for both fuels under stoichiometric conditions as well as for different fueling rates, for natural gas. Subsequently, the effects of oxygen enrichment are reviewed in terms of: volumetric efficiency, flame propagation velocity, coolant temperature, exhaust gas temperature, spark advance, brake mean effective pressure, brake horsepower. The use of oxygen enriched air results essentially in: 1 a significant increase in brake horsepower, 2 improved lean limits and reduction of spark advance, 3 a lower brake thermal efficiency.